1. Field of the Invention
The present invention relates to methods for changing the refractive index in elements comprising germanium silicate glass, and particularly to such methods in which laser radiation is employed for exciting a particular absorption band in the glass, such as one centered at 330 nm.
2. Description of the Prior Art
A Raman fibre laser is known emitting wavelength .lambda.=1.48 micron and comprising a fibre light guide based on SiO.sub.2 +GeO.sub.2 as the active medium, an ytterbium laser emitting in the wavelength 1.117 micron as the pumping source, and 5 Bragg fibre-optical gratings as the distributed reflectors for wavelengths of 1.175 micron, 1.24 micron, 1.31 micron, 1.40 micron and 1.48 micron that form, respectively, 5 resonators for 1st, 2nd, 3d, 4th and 5th Stokes components of Raman (stimulated combinational) scattering S. G. Grubb, T. Strasser, W. Y. Cheung, W. A. Reed, V. Mizhari, T. Erdogan, P. J. Lemaire, A. M. Vengsarkar, D. J. DiGiovanni, D. W. Peckham, B. H. Rockhey, "High-Power 1.48 Micron Cascaded Raman Laser in Germanium Silicate Fibres," Optical Amplifiers and Their Appl., Davos, Colo., USA, 15-17 June 1995, p. 197-199!.
The drawbacks of this laser is its complexity caused by the necessity to use five pairs of Bragg gratings and a relatively low efficiency of converting the radiation into 5th Stokes component. Furthermore, Bragg gratings do not have a sufficient depth of the refraction index modulation.
A Raman fibre laser emitting wavelength .lambda.=1.48 micron, comprising a fibre light guide based on SiO.sub.2 +GeO.sub.2 as the active medium, a solid body laser emitting in the wavelength of about 1 micron as the pumping source, and 6 Bragg fibre-optical gratings as the distributed reflectors for wavelengths of 1.117 micron, 1.175 micron, 1.24 micron, 1.31 micron, 1.40 micron and 1.48 micron that form, respectively, 5 resonators for 1st, 2nd, 3d, 4th, 5th and 6th Stokes components of Raman scattering, is known S. G. Grubb, T. Strasser, W. Y. Cheung, W. A. Reed, V. Mizhari, T. Erdogan, P. J. Lemaire, A. M. Vengsarkar, D. J. DiGiovanni, D. W. Peckham, B. H. Rockhey, "High-Power 1.48 Micron Cascaded Raman Laser in Germanium Silicate Fibres," Optical Amplifiers and Their Appl., Davos, Colo., USA, 15-17 June 1995, p. 197-199!.
The drawbacks of this laser is its complexity caused by the necessity to use six pairs of Bragg gratings, and a relatively low efficiency of converting the radiation into 5th Stokes component. Moreover, Bragg gratings do not have a sufficient depth of the refraction index modulation.
Another known Raman laser, comprises a fibre light guide based on SiO.sub.2 +GeO.sub.2 as the active medium, a neodymium laser emitting in the wavelength of 1.06 micron as the pumping source, and 3 Bragg fibre-optical gratings as the distributed reflectors for wavelengths of 1.117 micron, 1.175 microns and 1.24 micron that form, respectively, 3 resonators for 1st, 2nd and 3d Stokes components of Raman scattering S. G. Grubb, T. Erdogan, V. Mizhari, T. Strasser, W. Y. Cheung, W. A. Reed, P. J. Lemaire, A. E. Miller, S. C. Kosinski, G. Nykolak, P. C. Becker, D. W. Peckham, "1.3 Micron Cascaded Raman Amplifier in Germanium Silicate Fibres," Optical Amplifiers and Their Appl., Davos, Colo., USA, 3-5 August 1994, 187-190!.
The drawbacks of this laser is its complexity caused by the necessity to use three pairs of Bragg gratings, and a relatively low efficiency of converting the radiation into 3rd Stokes component.
An optical fibre is known that contains phosphorus to reduce the period of erbium ions relaxation and, as the result, the attenuation of the reverse energy transfer from erbium ions to ytterbium ions. U.S. Pat. No. 5,225,925 dated Jun. 7, 1995. IPC H 01 S 3/16!.
The drawback of this fibre is the impossibility to obtain radiation in the wavelengths of 1.24 micron and 1.48 micron owing to the presence of erbium ions in composition of the optical fibre.
A Bragg grating is known that is used as a distributed reflector and implemented in the form of a portion of a fibre light guide, the core refraction index of which light guide having been modulated U.S. Pat. No. 5,237,576 dated Jul. 8, 1995. IPC H 01 S 3/17!.
The drawback of this grating is its low efficiency for the reason that the chemical composition of the optical fibre core is not optimized.
A method is known for changing the refraction index in an optical waveguide of germanium silicate glass, inclusive of the step of acting on a fibre light guide along the optical axis by a laser radiation in the wavelength .lambda. approximately 480 nm K. O. Hill, Y. Fujii, D. C. Johnson and B. S. Kawasaki, "Photosensitivity in optical fibre waveguides: application to reflection filter fabrication," Appl. Phys. Lett. Vol. 32(10), 647-649 (1978)!. Here an argon laser having output power about 1 W and coherence lengthy about L=30 cm in length was used. In this known method the two-photon interaction takes place, i.e. a change in the refraction index was achieved when the absorption band of 240 nm was excited. In a fibre light guide an interference of the incoming and reflected from the face beams occurred, whereby a grating was formed in a light guide.
The drawback of this known method is a slight change of the refraction index .DELTA.n(.about.10) and the impossibility to vary the spacing of a grating being formed.
Another known method for changing the refraction index in an optical waveguide of germanium silicate glass includes the step of acting on a fibre light guide at an angle to the waveguide surface by a laser radiation having wavelength approximating 240 nm G. Meltz, W. W. Morey, W. H. Glenn, "Formation of Bragg gratings in optical fibres by a transverse holographic method," Opt. Lett., Vol. 14 (15), 823-825 (1989)!. In particular, in this known method, a change of the refraction index can be attained by action of radiation of second harmonic of an argon laser (.lambda.=244 nm), fourth harmonic of a neodymium laser (.lambda.=266 nm), an excimer laser based on KrF (.lambda.=248 nm), or a dye laser (.lambda.=240 nm). In the real practice an excimer laser is commonly used, which is the most unreliable, the most complex and expensive amongst the lasers mentioned above.
Radiation of the second harmonic of an argon laser has a sufficiently great output power (W.about.0.2 W) and coherent length (L.about.5 cm), therefore using the same a grating in a fibre light guide is formed by the interference effect directing two beams at angle .phi. to the light guide surface. However the resulting effective length is not sufficient to form a grating in the case when the radiation is directed along the axis of a fibre light guide due to a high absorption, which is impermissible. Changing angle .phi. the spacing of the grating being formed can be varied. By a similar method a change of the refraction index can be achieved by action of radiation of fourth harmonic of a neodymium laser (W.about.1 W), the coherent length of which is L.about.2-3 cm. Unfortunately, use of this laser is not efficient enough, as the radiation hits the edge of the band of absorption of germanium silicate glass, which is at a maximum of 240 nm. The powerful radiation from an excimer laser (W&gt;2 W) and the second harmonic of a dye laser that hit the center of the band of absorption of germanium silicate glass ensures a sufficiently great change of the refraction index (.DELTA..about.10.sup.-3). But radiation emitted by these lasers has a little coherent length (L&lt;1 mm), and that makes the use of the interference effect during formation of gratings rather difficult (the gratings are formed using special expensive and non-durable quartz masks, wherethrough the laser radiation is passed).
The main drawback of the aforementioned known method is the use of a too short-wave laser radiation resulting in degradation of the optical elements (that increases as the output power grows) and additional stimulated losses in the optical elements and fibre light guides (in particular, a wide band of absorption of fibre light guides, being 290 nm at most), and this prevents formation of extended and several gratings. Such gratings are not allowed to be passed by polymer claddings of the standard fibre light guides, which makes the formation of gratings therein difficult (the cladding must first be removed from the light guides). Moreover, the laser sources used in the prototype do not provide sufficient reliability.